In 39. Years as a flight engineer before ,CRM numerous times found myself being the third man out that provided necessary insight to defuse a hostel situation between Capt & Fo
This is the solution. A third deadheading pilot should be mandated. I'm sure he could be usefully engaged.
3rd crew isn't a panacea.
JAL Flight 350 plunged into Tokyo Bay when at 200' AGL on short final into Haneda, the Captain decided to commit suicide by pushing the yoke forward and activating the thrust-reversers.
The crash could not be prevented despite the FO desperately pulling back on the yoke while the Flight Engineer was physically restraining the Captain.